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error:
       … while evaluating an attribute name
         at /pwd/in.nix:5:10:
            4| in
            5|   attr.${key}
             |          ^
            6|

       error: expected a string but found a set: { }
       at /pwd/in.nix:5:10:
            4| in
            5|   attr.${key}
             |          ^
            6|